Evolving mobile cellular standards such as Global System for Mobile Communications (GSM) and Wideband Code Division Multiple Access (WCDMA) will likely require modulation techniques such as OFDM in order to deliver higher data rates. OFDM is a method for multiplexing signals which divides the available bandwidth (BW) into a series of frequencies known as sub-carriers.
In order to ensure a smooth migration from existing cellular systems to high capacity, high data rate systems using existing radio spectrum, new systems must be able to operate on a flexible BW. Third generation Long Term Evolution (LTE) has been proposed as a new flexible cellular system. LTE is intended as an evolution of the WCDMA standard. LTE will likely use OFDM and operate on BWs spanning from 1.25 MHz to 20 MHz. Data rates of up to 100 Mb/s will be possible in the high BW LTE service.
Low rate services such as voice are also expected to use LTE. Because LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), voice over IP (VoIP) will likely be the service carrying speech.
One important aspect of LTE is the mobility function. As a result, synchronization symbols and cell search procedures are of major importance in order for an apparatus, such as a user equipment (UE), to detect and synchronize with other cells.
The proposed cell search scheme for LTE is as follows:
1. Detect symbol timing for new cell using the primary synchronization signal (P-SyS). Furthermore, because there are three P-SyS, the UE also detects which of the P-SyS have been transmitted from the cell. The index of each P-SyS identifies the cell ID within a group. P-SyS is transmitted every 5 milliseconds (ms).
2. Detect frame timing and cell group using the secondary synchronization signal (S-SyS). The frequency domain representation of P-SyS is used as phase reference and then the S-SyS detection (correlation to different S-SyS sequences) is performed in the frequency domain.
3. From steps (1) and (2), the cell ID is detected.
4. Read broadcast channel (BCH) to receive cell specific system information
In LTE, there will be a possibility of using a short cyclic prefic (CP) or a long CP length. The short CP length (4.7 microseconds (μsec)) will be used for small cells and the long CP length (16.7 μsec) will be used for large cells and broadcast services. The intention is that the UE should detect the cell specific CP length blindly. This is preferably performed prior to detecting the frame timing and cell group using the secondary S-SyS (step 2 of the cell search scheme described above). Blind CP detection can be made in the time domain, as seen in the block diagram 100 of FIG. 1. In this case, the UE performs autocorrelation of the received signal with distance Tu corresponding to the OFDM symbol length. The correlation is summed and the power (absolute value) is calculated. Peaks 101A, 101B will arrive with a distance of Tu+Tg where Tg is the CP length. From that, the CP length can be computed at module 102. This time-domain approach is suitable for a single frequency, synchronized, network, such as digital video broadcasting-handheld (DVB-H), where signals from all cells are transmitted with the same CP length and are synchronized. However, this will typically not be the case in LTE. In LTE the cells can be operated in a asynchronus mode and different cells might have different CP lengths. This, in turn, will result in a risk of multiple correlation peaks making the time domain CP length detection much more complicated.
FIG. 2 shows the synchronization signal (SyS) structure 200 in LTE, for both the long CP 201 and short CP 202 case. A slot with a length of 0.5 ms in LTE consists of 7 OFDM symbols in the short CP case and 6 OFDM symbols in the long CP case. Every 10th slot, that is every 5 ms, the SyS is transmitted. For frequency division duplex (FDD) (full duplex) in LTE, P-SyS is transmitted in the last OFDM symbol in the slot and the S-SyS in the second to last OFDM symbol. For time division duplex (TDD), the S-SyS is transmitted in the last OFDM symbol and P-SyS is transmitted in the first OFDM symbol in the next slot.
It would be advantageous to have a low complexity blind CP detection method and apparatus that is robust also in scenarios existing in OFDM cellular system like LTE. The present invention provides such a method and apparatus.